Interesting following the r2ak thread in SA. Nice part of the world to cruise, lots of islands, grizzly bears and trees. Interesting the tactics for handling the tides.
Wondering what adv. and disadvantages an HP would have.
Interesting situation would be if your boat, say an HP had foils and there was a strong tide flow. Suppose it would help lift if tide against you.
Some posts mention having a few knots headwind because of the tide even when there is no actual wind, so the boat can still sail.
Sounds like the race is a lot of pedal power as well. I can only suppose that if it is very light weight you woukd be at an advantage in human propulsion results.

--------------------------------------------
On Thu, 21/6/18, Björn bjornmail@gmail.com [harryproa] <harryproa@yahoogroups.com.au> wrote:

Subject: Re: : Re: [harryproa] Rob's cheap wing sails
To: harryproa@yahoogroups.com.au
Received: Thursday, 21 June, 2018, 8:02 PM


 











| >How does twist help the need
for reefing?
|
|If the top of the wing/sail is
twisted to produce very little or no lift, it will add
little or no heeling moment. The bottom will produce the
lift for sailing. So this is similar to reefing. But with
the added drag from the unused sail/wing area.
Sure, but, why add the
complexity if twist, when one can just ease the angle of
attack?
Because of wind gradients. Wind is
always slower on the surface because of friction. Because of
this, the angle of the apparent wind might be significantly
different in the top compared to the bottom. So a sail
always needs to have twist. In lighter winds,a rigid wing
without twist might work. Then the top will work at a higher
angle of attack than the bottom. Maybe the top will lift and
the bottom will be at zero or negative angle of attack. The
boat will move, but it would be better if the whole sail
could provide lift.In the case of strong wind
sailing, if there is no twist and the top is the first part
of the wing to produce lift, it will create a strong heeling
moment, and the boat will be very sensitive angle of attack.
The boat might flip as soon as there is a small gust. With
twist/flex in a sail, the top will be flexible, and might
spill this gust. The bottom of the sail doesn't have the
same leverage, so the gust will not change the heeling
moment as much as at the top.
|
This is more or less what any sailor does when easing sheet
tension. However, easing sheet tension generally adds camber
though, which is the opposite to what one wants in a strong
breeze.. On the foresail, there is the option to change the
angle of the sheet in this situation, to make the bottom
part tight and flat, and the top part twisted and spilling
wind or even flogging. For the main, tightening the outhaul
can flatten the bottom of the sail to get a similar
effect.
Sure,
but are we not talking about Robs wing sail? Which has no
fore sail? This is a schooner or una rig, right? Does one
not generally ease said sail by decreasing the angle of
attack?


Most boats
have a bermudan rig, so I used it as a reference, and I was
saying that it has many ways to control the 3D-shape of the
sails. All types of sails needs ways to control camber,
angle of attack, and twist, to work on a sailing boat, I
think. (Maybe not some downwind sails)
You are right that the harryproa
sails with a wishbone boom doesn't change the twist with
the sheet, like most other boats. The sheet only changes
angle of attack of the boom, so it works like the traveler
on a catamaran. Other ropes control twist and camber, but
are not constantly controlled, because the flexible mast
changes angle of attack/camber on the top of the sail
somewhat automatically. I think you can compare it to a
spring controlled flap on a wing. If the spring has the
right rate and pre tension it can give the wing almost
constant lift independent of speed. So if the mast flex rate
and the tension of the twist control ropes fits the
conditions and RM of the boat, it should be very forgiving
in gusts. Because the heeling moment from the top of the
sail should be constant, so the sailor only has to adjust
for changes in lift of the bottom of the sail. And if he
isn't flying the hull, there is likely enough margin
that he doesn't have to adjust the sheet at when the
gust hits.


|
In any situation though, if the boat is steered in a
direction which increases the angle of attack significantly,
it will heel over anyway. A wing with a tail should solve
that situation, since the tail works to keep the apparent
wind at a constant angle of attack.
I
don't think Rob's wing sail is expecting to have a
tail. But yeah, a tail makes sense on a solid wing
sail.
Rob has at least once indicated a
desire to have a tail. I think multihulls are generally hard
to trim because the sudden changes in apparent wind angle
due to the fast acceleration and large speed range. The
sailor really have to sheet a lot and develop a great
understanding of how boat speed affect the apparent wind
angle to avoid stalling the sail or heeling too much. So I
think it makes a lot of sense with a tail which does a lot
of the work automatically.
|
Another problematic situation is accelerating to a speed
where the lift is enough to heel the boat over. This is
where the sailor has to change his input in some way, even
with a wing with a tail. Perhaps by changing the angle of
the tail, or by decreasing angle of attack in the top
(twisting the flap and/or wing).
 Should
the lift not drop as you gain speed?
 Lift is
proportional to the square of the apparent wind speed, so
higher speed has the potential to increase the forces
significantly. But the apparent wind angle will decrease
when the boat accelerates, so generally lift is reduced.
However, when I'm sailing, I think often part of the
sail doesn't have attached flow. It's easy to stall
some part of the sail. When the boat accelerates, and the
angle of attack decreases, flow might attach, and the sail
suddenly develops a lot of lift in this now higher apparent
wind. This leads to heeling and I have to release the sheet
completely to prevent a capsize, or in a better case, ease
the sheet a little, or steer a little, to find a good
balance, and reach a great
speed. Of
course one can change the angle of attack. Is that not
easier than changing twist? And if so, I am back to why have
twist control?
Depending on conditions, the wind
gradient will be different. Depending on point of sail and
speed, the resulting gradient in apparent wind angle will be
different.
On Thu, Jun 21, 2018 at
12:56 PM, '.' eruttan@yahoo.com
[harryproa] <harryproa@yahoogroups.com.au>
wrote:















 













| >How does twist help the need for reefing?

|

|If the top of the wing/sail is twisted to produce very
little or no lift, it will add little or no heeling moment.
The bottom will produce the lift for sailing. So this is
similar to reefing. But with the added drag from the unused
sail/wing area.



Sure, but, why add the complexity if twist, when one can
just ease the angle of attack?



| This is more or less what any sailor does when easing
sheet tension. However, easing sheet tension generally adds
camber though, which is the opposite to what one wants in a
strong breeze. On the foresail, there is the option to
change the angle of the sheet in this situation, to make the
bottom part tight and flat, and the top part twisted and
spilling wind or even flogging. For the main, tightening the
outhaul can flatten the bottom of the sail to get a similar
effect.



Sure, but are we not talking about Robs wing sail? Which
has no fore sail? This is a schooner or una rig, right?
Does one not generally ease said sail by decreasing the
angle of attack?



| In any situation though, if the boat is steered in a
direction which increases the angle of attack significantly,
it will heel over anyway. A wing with a tail should solve
that situation, since the tail works to keep the apparent
wind at a constant angle of attack.



I don't think Rob's wing sail is expecting to have a
tail. But yeah, a tail makes sense on a solid wing
sail.



| Another problematic situation is accelerating to a speed
where the lift is enough to heel the boat over. This is
where the sailor has to change his input in some way, even
with a wing with a tail. Perhaps by changing the angle of
the tail, or by decreasing angle of attack in the top
(twisting the flap and/or wing).



Should the lift not drop as you gain speed?

Of course one can change the angle of attack. Is that not
easier than changing twist? And if so, I am back to why
have twist control?



| I read somewhere that the AC50 cats could twist the top
enough to produce negative lift, to actually get righting
moment from the top of the wing.



The bell shaped wing guys are big on this. For flying
machines, it allows the control of yaw while turning without
a vertical tail (flying wings). Al Bowers calls it the bird
wing solution, because that's how birds fly. No
vertical tail.



For a boat though, I still do not understand why the added
complexity is desirable. Yes, a small thrust at the top of
the mast is super neat. But I am not sure how it works when
I am expecting the mast to flex when gusting, and it stands
rigid due to aerodynamic loading. That seems like it
removes a safety feature.



Thanks for being patient with a sailing noob Björn

































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